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  The domestication of wild species led directly to denser human populations by yielding more food than the hunter-gatherer lifestyle could provide. In societies that possessed domestic animals, livestock helped to feed more people by providing meat, milk, and fertilizer, and by pulling plows. Large domestic animals became the societies’ main source of animal protein, replacing wild game, and they also furnished wool, leather, and land transport. Humans have domesticated only a few species of large animals, with “large” defined as those weighing over 100 pounds (45 kilograms). Fourteen such species were domesticated before the twentieth century, all of them terrestrial mammals and herbivores. The five most important of these are sheep, goats, pigs, horses, and cattle or oxen.

  Small animals such as ducks, geese, rabbits, dogs, cats, mink, bees, and silkworms have also been domesticated. Many of these small animals provided food, clothing, or warmth. However, none of them pulled plows or wagons, none carried riders, and none except dogs pulled sleds. Furthermore, no small domestic animals have been as important for food as have large domestic animals.

  Early herding societies quickly domesticated all large mammal species that were suitable for domestication. There is archaeological evidence that these species were domesticated between 10,000 and 4,500 years ago, within the first few thousand years of the origins of farming-herding societies after the last Ice Age. The continent of Eurasia has been the primary site of large mammal domestication. Having the most species of wild mammals to begin with, and losing the fewest to extinction in the last 40,000 years, Eurasia has generated the most candidates for domestication.

  Domestication involves transforming wild animals into something more useful to humans. Truly domesticated animals differ in many ways from their wild ancestors. These differences result from two processes: human selection of individual animals that are more useful to humans than other individuals of the same species, and evolutionary responses of animals to the forces of natural selection operating in human environments rather than in wild environments.

  To be domesticated, a wild species must possess several characteristics. A candidate for domestication must be primarily a herbivore because it takes less plant biomass to feed a plant eater than it does to feed a carnivore that consumes plant eaters. No carnivorous mammal has ever been domesticated for food simply because it would be too costly. A candidate must not only weigh an average of over 100 pounds but also grow quickly. That eliminates gorillas and elephants, even though they arc herbivores. Moreover, candidates for domestication must be able to breed successfully in captivity.

  Since almost any sufficiently large mammal species is capable of killing a human, certain qualities disqualify a wild animal for domestication. The animal cannot have a disposition that is nasty, dangerous, or unpredictable—characteristics that eliminate bears, African buffaloes, and some species of wild horses. The animal cannot be so nervous that it panics around humans. Large herbivorous mammal species react to danger from predators or humans in different ways. Some species are nervous, fast, and programmed for instant flight when they perceive danger. Others are less nervous, seek protection in herds, and do not run until necessary. Most species of deer and antelope are of the former type, while sheep and goats are of the latter.

  Almost all domesticated large mammals are species whose wild ancestors share three social characteristics: living in a herd, maintaining a dominance hierarchy in the herd, and having herds that occupy overlapping home ranges instead of mutually exclusive territories. Humans have taken advantage of these characteristics in keeping domestic animals together with others of their species and in close proximity to other species of domestic animals.

  Glossary:

  terrestrial: living on land rather than in water

  herbivores: animals that feed mainly on plants

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